(a) Field of the Invention
The present invention pertains to compound field effect transistor comprised of a combination of two junction-type field effect transistors and presenting a saturated characteristic (drain current-drain voltage characteristic) resembling the output characteristic of a pentode vacuum tube.
(B) Brief Description of the Prior Art
In the past, there has been proposed a compound field effect transistor which is comprised of horizontal junction-type field effect transistors having a saturated characteristic (drain current-drain voltage characteristic) resembling the output characteristic of a pentode vacuum tube, one of which transistors serving as an input transistor and the other as an output transistor which are connected together in cascode fashion (refer to Nikkei Electronics Journal, September, 1972 issue).
Said horizontal junction-type field effect transistor is of the type that the flow of the majority carriers passing through the channel region defined in the horizontal direction along the surface of the semiconductor substrate is controlled by the pn-junction gate provided so as to oppose said channel region, and this field effect transistor exhibits a saturated characteristic resembling the output characteristic of a pentode vacuum tube.
This compound field effect transistor will exhibit a high transconductance gm, a high breakdown voltage and a superior high frequency characteristic (due to decrease of the Miller effect capacitance of the input transistor), if the transconductance gm of said input transistor is sufficiently high and if the breakdown voltage of the output transistor is sufficiently high.
In such a known compound field effect transistor, a horizontal junction-type field effect transistor is used for the output transistor of the compound field effect transistor as stated above. Accordingly, it is extremely difficult to elevate the breakdown voltage to a level over 250 volts. Also, the horizontal junction-type field effect transistor has the drawbacks that, when it is intended to elevate its breakdown voltage, the build-up characteristic of the drain current up to the point of saturation will lose sharpness, and there will tend to appear an intensified variance of the drain current I.sub.DS at the absence of a gate-source bias voltage and also of the pinch-off voltage. For this reason, the known compound field effect transistor has been required to sacrifice its other characteristics in order to elevate its breakdown voltage.
Also such a known compound field effect transistor as discussed above desirably is such that the whole device is formed as a monolithic integrated circuitry from the viewpoints of a higher yield in manufacture, an improvement in reliability, a reduction in size, weight and cost. However, a horizontal junction-type field effect transistor having a high breakdown voltage and a horizontal junction-type field effect transistor having a high transconductance gm are markedly different from each other in the manufacturing conditions, and accordingly there two cannot be formed into a monolithic integrated circuitry. For this reason, known compound field effect transistor of this type has been arranged in the form of the so-called hybrid integrated circuitry.